Many characteristics of paper conventionally employed in printing processes are unstable. One set of such characteristics is the dimensions of the paper, and in particular the length and width. These dimensions change as functions of heat, moisture, and tension, for example, creating potential difficulties in registering repeated patterns appearing on two or more webs which, at some point, have been processed differently.
Historically, dimensional instability of paper has not caused any significant problem for the printing industry. This is because the problem largely can be avoided merely by simultaneously printing and finishing multiple webs, thereby subjecting the webs to essentially the same heat, moisture, tension, and other environmental and equipment-related factors. A recognized drawback, of course, is the limited number of colors available to printing on the webs, as use of the available print stations must be divided to service the two (or more) webs.
Prior attempts to resolve this issue have involved continual varying of the tension on the webs throughout the finishing line. In other words, variably-increased tension is applied throughout the process to cause the shorter or narrower patterns on the webs to match the longer or wider ones. While theoretically capable of matching certain characteristics of webs, these tension-oriented attempts typically have been disadvantageous, in that they disrupt aspects of the finishing process such as (but not necessarily limited to) folding, gluing, and die-cutting of the webs.